Conventionally, an optical device composed of glass or the like is subjected to surface treatment for reducing return light by surface reflection and for increasing transmitted light. As a specific method of this surface treatment, there is known a method of forming fine and dense depressions and elevations on a surface of the optical device.
In a case of thus forming the depressions and elevations in a periodic pattern on the surface of the optical device, light is diffracted when the light transmits through the surface of the optical device to greatly reduce straight components of the transmitted light. However, when the depressions and elevations formed on the surface of the optical device is formed in a rectangular shape in such a manner that a pitch thereof is shorter than a wavelength of the transmitting light, the light is not diffracted. Therefore, it is possible to obtain an anti-reflection effect effective to the light with a single wavelength corresponding to the pitch, a depth or the like of the depressions and elevations.
Furthermore, it is known to be able to obtain an anti-reflection effect also to light with a wide range of wavelength by forming the depressions and elevations to be not in a rectangular shape, but in a so-called conical shape (conical pattern) in which the ratio of the crest-side volume by the material of the optical device to trough-side volume by air continuously varies (for example, refer to Patent Document 1 and Patent Document 2).
For carrying out a structure with anti-reflection to such a wide range of wavelengths, a fine pattern having less than the wavelength is required. Accordingly, there is known a method of using an electronic beam lithography technology for producing such a fine structure. This method is a method in which after coating a substrate with electron resist, the electron beam is used to perform patterning thereon, and reflective etching is used to etch the substrate.
Furthermore, it is known that organic colloid is used to produce a nano periodic structure, thus obtaining an anti-reflection structure (for example, refer to Patent Document 3). The producing method using the organic colloid is a method in which the organic colloid is mixed into a solution, the colloid is coated on the surface of the substrate and a structure is produced by reflective etching based upon colloid beads in such a manner as to form a fine structure with a wavelength or less as a target, thus forming the anti-reflection structure.    Patent Document 1: Japanese Patent Application Laid-Open No. 2001-272505    Patent Document 2: Japanese Patent Application Laid-Open No. 2006-243633    Patent Document 3: Japanese Patent Application Laid-Open No. 2005-331868